1. Field of the Invention
The present invention relates to solid oxide fuel cells, and in particular planar solid oxide fuel cells with electrode material supported on a support having integrated in its structure passageways for distribution of cell reactant gases.
2. Description of the Related Art
Presently, solid oxide fuel cells (SOFC) are mainly produced in two different designs in form of tubular types and flat planar types. The different fuel cell designs possess a number of advantages and disadvantages such as easy gas manifolding and structural reliability which has been demonstrated for the tubular types, whereas compactness and potential reduction of materials involved are the main advantages of the flat planar design. Very low voltage losses due to low area specific internal resistance can be obtained with the flat planar cell is stacked.
Recent development of thin supported electrolytes for the flat planar cell type in connection with improved electrode performance allows to lower the operation temperature from previously required 1000xc2x0 C. operation temperature, which was typical for self-supported electrolyte cells, to a operation temperature of about 800xc2x0 C. without any reduction in overall cell performance measured as area specific electrical power. Reduction of operation temperature makes it possible to use metallic bi-polar separation plates between the cells. Furthermore, metallic construction materials for the manifolds and heat exchangers leading to improved system reliability and reduced system price can be introduced. Examples of SOFC with thin electrolytes supported by the anode or cathode of the cells are disclosed in the literature (cf. Advances in the Anode Supported Planar SOFC Technology; H. P. Buchkremer et al., Electrochemical Proceedings, Vol. 97-18 (1997); Thin Anode Supported SOFC; S. Primdahl et al., Electrochemical Society Proceedings, Vol. 99-19 (1999); and, Advances in Tubular Solid Oxide Fuel Cell Technology; S. C. Singhal, Electrochemical Society Proceedings, Vol. 95-1 (1995)). Especially, the anode-supported thin electrolyte flat plate cell design has some important advantages including low contact resistance between the electrolyte and the anode in addition to simplified and cheaper manufacturing methods based on co-firing of cell components.
The anode structure is usually produced by tape casting a mixture of NiO and zirconia powder with a layer thickness of about 0.1 to 1 mm. A thin electrolyte layer is applied for on this anode support layer by spray moulding, dip moulding, layer casting or electrophoretic deposition. The thickness of the thin electrode is typically 10-50 xcexcm. During the final firing of the thus obtained multi-layer structure, the anode layer becomes porous with an open porosity in the range of 30-60%, whereas the electrolyte layer densities to a gas-tight material, while a three-phase boundary (anode-electrolyte-porosity) is established in which the anode electro catalytic reaction takes place. Reaction rate of electrode reactions is limited by transport of fuel gas and gaseous reaction products through the porous anode structure. The characteristics of the three-phase boundary and the anode porosity therefore take part in the total voltage polarisation of the cell. One solution to this problem is to subdivide the anode into a thin active anode layer with a typical thickness of 1-50 xcexcm and an anode support and current collecting layer with a typical thickness of 50-1000 xcexcm, basically consisting of the same mixture of NiO and zirconia. However, this configuration still fails to fulfil a required effective gas supply through the porous open structure together with a highly mechanically stable support for the electrolyte layer. Furthermore, problems with effective mechanical and electrical contact between the porous anode support layer and the bi-polar plates containing the anode gas supply channels still remain to be solved. A number of attempts have been made in the past to improve electrical contact by application of nickel felt in between the anode support layer and the metallic bi-polar plate. Gas supply channels may be formed by machining grooves into the solid metallic bi-polar plate. The above solutions are expensive and not useful in commercial use of SOFC technology.
In general, this invention is an improved SOFC, which integrates the gas supply channels with the cell support and the anode layer. Furthermore, the inventive SOFC makes it possible to produce large cell areas based on cheap manufacturing and materials.